The Ovonic EEPROM is a novel, proprietary, high performance, non-volatile, thin-film electronic memory device. Its advantages include non-volatile storage of data, potential for high bit density and, consequently, low cost because of its small footprint and simple two-terminal device configuration, long reprogramming cycle life, low programming energies and high speed. The Ovonic EEPROM is capable of both analog and digital forms of information storage. Digital storage can be either binary (one bit per memory cell) or multi-state (multiple bits per cell). Only minor modifications are necessary to switch between the two digital modes. For purposes of the instant invention, the terms "memory elements" and "control elements" will be employed synonymously.
The general concept of utilizing electrically writable and erasable phase-change materials (i.e., materials which can be electrically switched between generally amorphous and generally crystalline states) for electronic memory applications is well known in the art, as is disclosed, for example, in U.S. Pat. No. 3,271,591 to Ovshinsky, issued Sep. 6, 1966 and in U.S. Pat. No. 3,530,441 to Ovshinsky, issued Sep. 22, 1970, both of which are assigned to the same assignee as the present invention, and both disclosures of which are incorporated herein by reference.
As disclosed in the '591 and '441 patents, such phase-change materials can be electrically switched between structural states of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. That is, the '591 and '441 patents describe that the electrical switching of such materials is not required to take place between completely amorphous and completely crystalline states but rather can be in incremental steps reflecting changes of local order to provide a "gray scale" represented by a multiplicity of conditions of local order spanning the spectrum between the completely amorphous and the completely crystalline states. The early materials described by the '591 and '441 patents could also, if required, be switched between just the two structural states of generally amorphous and generally crystalline local order to accommodate the storage and retrieval of single bits of encoded binary information.
The electrically erasable phase-change memories described in the '591 and '441 patents had a number of limitations that prevented their widespread use. Specifically, the following represent the most significant of these limitations: (i) a relatively slow (by present standards) electrical switching speed, particularly when switched in the direction of greater local order (in the direction of increasing crystallization); (ii) a relatively high input energy requirement necessary to initiate a detectable change in local order; and (iii) a relatively high cost per megabyte of stored information (particularly in comparison to present hard disk drive media).
The phase-change materials described in the '591 and '441 patents were based on changes in local structural order. The changes in structural order were typically accommodated by atomic migration of certain species within the material. Such atomic migration between the amorphous and crystalline states required a time necessary to accommodate the migration, thereby making the switching times and energies relatively high. Materials having reduced atomic migration and thus decreased electrical switching times and energies were disclosed in U.S. Pat. No. 5,166,758 to Ovshinsky, the disclosure of which is incorporated by reference herein. The memory material of the present invention further reduces the electrical switching times and energies of the Ovonic EEPROM.